The present disclosure is related to rotating machines such as electric motors. More particularly, the disclosure is related to structures for bearings used to rotatably support rotors of rotating machines in housings.
Rotary machines such as electric motors and pumps have stationary components, e.g., a stator, and a rotating component, such as a rotor on a rotor shaft. In electric motors such as AC induction motors, the stator may include a selected number of steel laminations, typically with a thickness of 0.5 mm or less. Electrical windings pass through shaped openings in the laminations, and the center of the laminations may form a bore in which the rotor is inserted. The rotor may be supported in bearings which are fitted to support the rotor shaft. In the case of an induction motor, the rotor shaft may carry laminations and electrically conductive bars, or in the case of a permanent magnetic motor, the rotor shaft may carry permanent magnets.
For a given electromagnetic design, the power of the rotary machine is generally proportional to the cross-sectional area of the machine (which is proportional to the square of the diameter of the machine), the length of the machine, and the speed of the rotating shaft. Electric motors used in well bores are generally constrained by the internal diameter of well bore conduit such as casing and production tubing. For example, a well bore electric motor may be 76.2 mm (3 inches) to 177.8 mm (7 inches) in diameter, but may be as small as 44.5 mm (1.75 inches) in diameter.
For some rotary machines, such as well bore electric motors, the length of the machine may be large compared to the diameter of the machine in order to fit within the internal diameter of the well bore conduit but still produce sufficient power for the intended purpose of the motor, such as fluid pumping. As an example, the length of a well bore electric motor may be on the order of 3 to 6 meters, even with appropriate measures taken to minimize the length of the motor. Such a long motor may require a plurality of bearings to be installed at spaced apart positions along the rotor shaft to rotatably support the rotor shaft. Typically one bearing is provided about every ¼ meter along the rotor shaft, resulting in typically 12 to 25 bearings in a typical rotor shaft. This presents a challenge for assembling the motor.
As practical matter, it is advantageous to use a small diameter machine at high rotary speeds in order to deliver as much useful power as possible without the machine becoming excessively long. The speed at which a rotary machine can operate reliably is determined in part by the stability of the rotating parts. Critical factors in rotating component dynamic performance are rotating mass, rotor shaft stiffness, bearing separation, bearing radial stiffness and bearing damping. Rotating mass is generally fixed by electromagnetic design considerations. Rotor shaft stiffness is principally related to the rotor shaft diameter, which generally cannot be increased for any selected external diameter motor without compromising the electromagnetic design. Small bearing separation will reduce the unsupported length of shaft and increase stability. However, it is generally preferable that bearing separation is as large as possible to keep the number of bearings to a minimum and thereby avoid excessive cost, friction, drag, and manufacturing complexity.
For small diameter, long rotary machines, such as well bore electric motors, where the shaft diameter is limited and shaft stiffness is impracticable to increase by increasing the rotor shaft diameter, bearing radial stiffness and damping become limiting factors that determine the safe operating speed of the machine. In such rotary machines, bearings that provide greater radial stiffness are desirable to maintain the rotor shaft stability at high rotational speeds.